A super luminescent diode (hereinafter may be abbreviated as SLD) is a semiconductor light source which is capable of providing a relatively high light output of 1 mW or more, like a semiconductor laser, while having a wide spectrum distribution, like a light-emitting diode. Because of its characteristics, the SLD attracts attention in the fields of medicine and measurement where high resolution is required. For example, the SLD is used as a light source for an optical coherence tomography (OCT) apparatus capable of obtaining tomographic images of living tissue.
NPL 1 discloses an SLD having a multi-electrode structure that includes a plurality of electrode pairs in a single SLD device. In the SLD disclosed in NPL 1, a region including an exit end emits light of first-order level and ground level, so that a large peak appears on the lower-wavelength side and a small peak appears on the longer-wavelength side. In this SLD, on the other hand, a region adjacent to the region including the exit end emits light of ground level. This light is combined with that from the region including the exit end described above, so that light output from the SLD has a wide emission wavelength range.